Every Mahapurana states that Vishnu Purana has 23 thousand verses but if we count the actual no. of verses there are only around 7000. So about 16 thousand verses are missing.
I found another purana, the Vishnudharmottara, on Exotic India which appears related to Vishnu Purana. So I have the following questions:
Is Vishnu Purana a composite of 2 or more puranas?
Was Vishnudharmottara originally the second part of Vishnu Purana?